Some implantable medical devices, such as pacemakers, defibrillators, and cardioverters (collectively referred to as implantable cardiac stimulating devices) are designed to monitor and stimulate the cardiac tissue of patients who suffer from cardiac arrhythmias. Using leads connected to the patient's heart, a cardiac stimulating device typically stimulates cardiac tissue by delivering electrical pulses in response to measured cardiac events which are indicative of a cardiac arrhythmia. Properly administered therapeutic electrical pulses often successfully reestablish or maintain the heart's regular rhythm. Implantable cardiac stimulating devices can treat a wide range of cardiac arrhythmias by using a series of adjustable parameters to alter the energy content, the shape, the location, and the frequency of the therapeutic pulses. The adjustable parameters are usually stored in a memory of the implantable device. The adjustable parameters can be defined or altered telemetrically by a medical practitioner using an implantable device programmer, such as the one disclosed in the copending, commonly-assigned U.S. Pat. No. 5,724,985, issued Mar. 10, 1998, entitled "Improved User Interface for an Implantable Medical Device Using an Integrated Digitizer Display Screen", now U.S. Pat. No. 5,724,985, which is hereby incorporated by reference in its entirety.
After initial implantation and configuration of the implantable medical device, the medical practitioner typically performs periodic follow-up examinations to determine if the device is operating properly. On occasion, it may also be desirable for the medical practitioner to alter the device settings to improve the performance of the device. In order to make appropriate decisions regarding such changes, the medical practitioner should consider not only the current condition of the patient and the implantable medical device, but also historical information representative of changes to the patient's condition and changes in the operational characteristics of the implantable medical device.
Historical medical data should be taken into account because, while most implantable medical devices serve patients for years, certain physiologic and parametric changes having an impact on the performance of the device may occur during the time of service. These changes may include changes in characteristics of the patient's cardiac disorder, changes in the patient's course of drug therapy, changes in the patient's overall health, as well as changes in the operational characteristics of the implantable medical device (such as lead impedance and remaining battery life). In addition, medical knowledge about cardiac disorders and preferred methods of treatment may advance during the time of service.
Several approaches to gathering and presenting historical medical data have been developed to address the medical practitioners' needs. One such system for compiling and storing historical medical data measured by the medical practitioner during previous follow-up visits and medical data acquired between visits by the implantable device, is disclosed in the copending, commonly-assigned U.S. Pat. No. 5,722,999, issued Mar. 3, 1998, entitled "A System and Method for Storing and Displaying Historical. Medical Data Measured by an Implantable Medical Device", now U.S. Pat. No. 5,722,999, which is hereby incorporated by reference in its entirety.
While a significant amount of historical medical data is gathered during follow-up visits, often the most important events indicative of a patient's condition and the implantable device's performance occur between the follow-up visits. For example, most patients experience the majority of their cardiac arrhythmia episodes during the months between follow-up visits. In order to improve the medical practitioner's access to medical data representative of the patient's condition and of the performance of the implantable device, it was desirable to develop a technique for recording and storing medical data between follow-up visits.
In recent years, several approaches have been developed to enable implantable devices to record and store medical data between follow-up visits. The first approach to recording inter-visit medical data involves simply recording all medical data into the implantable device storage memory up to that memory's capacity. The second approach to recording inter-visit medical data, involves using a circular buffer to store the data. A circular buffer is a memory management scheme which stores a certain amount of data and discards a proportional amount of old data when a certain amount of new data is received. Thus, if a circular buffer has a capacity of storing twenty data items, the twenty first data item stored in the buffer would overwrite the first data item, and so on. Neither the direct memory storage approach, nor the circular buffer approach, provides the medical practitioner with the most important form of inter-visit medical data--medical data recorded during a cardiac episode (e.g., arrhythmia, a series of Premature Ventricular Contractions (PVCs)), and during the performance of an implantable device function (e.g., mode switching). Thus, it would be desirable to enable the implantable device to record and store inter-visit medical data recorded during a cardiac episode and during the performance of an implantable device function.
This need is partially addressed by a third approach to recording inter-visit medical data. The third approach to recording inter-visit medical data allows the patient to trigger recording of the data via some form of a remote control device linked to the implantable device. For example, if the patient is feeling uncomfortable, the patient may initiate the recording function so that at the next follow-up visit the medical practitioner may view and analyze the medical data recorded by the medical device during the incident of the patient's discomfort. However, this approach has a limitation in that the recording of the cardiac data is left to the patient's discretion and ability to trigger it.
Thus, it would be desirable for the medical practitioner to define what inter-visit medical data is important enough to be recorded and stored. It would also be desirable for the implantable device to automatically record and store inter-visit medical data which is defined as important by the medical practitioner. It would further be desirable to record inter-visit medical data both before and after an important cardiac episode is detected and before and after a performance of an implantable device function.